Obstructive sleep apnea (OSA) occurs when tissue in the upper airway blocks the airway during sleep. The brain will sense the rise in CO2, and will wake up the person so that breathing resumes. Such an event is called an apnea. A partial airway blockage causing an awakening is called a hypopnea. A person is unlikely to remember such awakenings, but sleep is disrupted. The severity of obstructive sleep apnea is measured by the frequency of awakenings, as shown in the table below.
Apneas + Hypopneas/HourOSA Classification0-5Normal 5-15Mild15-30Moderate30+Severe
Untreated, OSA not only leaves patients chronically fatigued, but it also carries significant health consequences. Unfortunately, despite a very high prevalence of OSA in the population, the vast majority of sufferers remain undiagnosed. Some estimates put the number of people with obstructive sleep apnea who are undiagnosed as high as 85%. In the US alone, this could mean 35 million undiagnosed sufferers. This large undiagnosed population is one of the biggest public health issues in sleep medicine.
There are many reasons for the low rate of diagnosis. The field of sleep medicine is relatively new. Awareness of clinical sleep problems and their causes remains relatively low. The understanding of sleep-disordered breathing is still advancing at a consistent rate, and will increase awareness further as more clinical consequences are characterized. Further, among some of the population there is a misconception that sleep-disordered breathing is not a serious condition, and is often dismissed as simple snoring. Awareness of the condition is presently significantly lower than the severity warrants.
A further impediment to diagnosis is the relatively onerous process through which most sufferers must pass to get diagnosed. Typically, an initial physician visit is followed by a second visit to a sleep specialist, and then an in-lab polysomnogram (PSG). Polysomnography is a very thorough observation while the user is sleeping. It is conducted in a laboratory setting, with a minimum of 22 electrodes and sensors placed on the patient, and with observation via video throughout the night. Many patients find this foreign sleep environment and observation inconvenient and intrusive. The very nature by which the data is gathered interferes with the typical, natural sleep habits of the test subject. Many labs also have long wait times for studies, especially for weekend studies, which are preferred by some patients to try to reduce the amount of disruption to their work routine. In-lab PSG studies also carry a high cost, often $2000-3,000 per night, a significant burden to the healthcare system. More recently, home sleep testing (HST) devices have been utilized to bring 4-8 channels of the traditional lab test into the home environment. Unfortunately, they still require a prescription, are not simple to use, and result in insufficient data about 20% of the time due to incorrectly placed or inadequately secured electrodes.
Some sufferers report not wanting to go through the whole process of the PSG only to find out the bad news of an apnea diagnosis. This form of wishful denial hinders the rate of diagnosis and postpones the initiation of treatment and its benefits. Many sufferers persist for years, suspecting they might have sleep apnea, but not acting to undertake the testing. This results in significantly increased healthcare spending, and reduced quality of life. Studies have calculated the cost of increased healthcare utilization by undiagnosed sleep apnea sufferers between $2000 and $12,000 additional USD per year. One study estimated the total economic cost of OSA (diagnosed and undiagnosed) to be between $65 and $165 billion annually. There are many impacts of sleep apnea that are difficult to fully estimate, such as the effect on interpersonal relationships of the sufferer.
In addition to these shortcomings, the PSG also has the challenge of trying to understand a complex patient given only one night of data. Due to the inconvenience and expense of PSG, most patients are only studied for one night. However, sleep quality can vary tremendously on a night-to-night basis. Many factors can influence sleep differently each night: exercise, diet, alcohol, stress, environment. It is a significant limitation of the PSG to only gather one night of data. Ideally, data would be gathered from each night of sleep throughout the typical fluctuations by these factors to more completely characterize a patient's sleep.
For those who do get diagnosed, the frontline therapy is Positive Airway Pressure (PAP). It is also called Continuous Positive Airway Pressure (CPAP), or Automatic Positive Airway Pressure (APAP). This disclosure refers to all such therapies as “PAP.” PAP is the most widely used and the most effective treatment for OSA. In PAP, a bedside compressor supplies pressurized air to the patient's airway through a hose and mask. The air pressure is set sufficiently high to maintain an open airway during sleep. The clinical data show significant benefit to the use of PAP therapy. Many patients report significant positive impact on their health and quality of life. However, many patients do not feel the benefit of PAP therapy all the time. This can lead to a patient perception that the treatment isn't effective, despite significant clinical evidence to the contrary. Some such patients then discontinue therapy, or only use it sporadically. The clinical data show that consistent use of PAP therapy yields the most benefit. Unfortunately, for these sporadic users, there is no convenient way to determine if the therapy is actually working, beyond user perception.
There is another category of patient who will use PAP therapy consistently for a while, then discontinue use once feeling good, assuming that they might be cured, or no longer need PAP treatment. Shortly thereafter they will again feel the fatigue and cognitive impairment reminiscent of their time before PAP therapy. Once they recognize this, they return to therapy. Some healthcare providers have termed this the yo-yo group, as they vacillate on and off therapy like a toy yo-yo going up and down.
Unfortunately for all PAP users, there is no convenient way to ascertain in a scientifically sound manner the effect of using and not using their PAP device on a nightly basis.
Sleeping position plays a very significant and greatly underappreciated role in Obstructive Sleep Apnea. When sleeping in a supine position, gravity's effect on the tongue, tonsils, soft palate and other structures of the upper airway is more pronounced. Clinical studies have shown that the majority of sleepers have at least twice as many blockages when back sleeping compared to side sleeping. The blockages that occur while supine sleeping are more severe: they last longer and result in more significant dips in blood oxygen levels. For some patients, the ratio of supine Apnea-Hypopnea Index (“AHI”) to lateral AHI exceeds 4 to 1. Sleeping position is one of the greatest determinants of the likelihood of an apnea or hypopnea occurring. Consequently, sleeping position is also one of the strongest determinants of PAP pressure needed to maintain a patent airway. During an in-lab titration study, PAP pressure settings are determined during REM stage, supine position sleeping. This is because during REM stage, supine sleeping the airway is most prone to collapse. Thus, PAP pressure settings are classically set to cover the worst-case scenario conditions. In fact, a titration study is not considered successful unless REM supine sleep is present. During some studies, technicians will enter the room and push the patient onto their back in order to capture REM supine sleep and the related pressure setting.
Studies have shown that a patient's required PAP pressure is significantly lower when not sleeping supine. One study found that the required pressure during side sleeping for all patients, not just those with positional OSA, was almost 3 cm H2O lower. For those with positional OSA, the pressure difference between back and side can be much larger.
Studies are not required for most people to recognize the role of sleeping position on breathing. For centuries, it has been reported that people sleep louder when supine, and common advice suggests elbowing a loud snorer to get them off their back to quiet down.
Despite all this evidence, the role of sleeping position in OSA remains a mystery to the vast majority of OSA sufferers. Very few sufferers understand what role sleeping position plays in their apnea.
Accordingly, there is a compelling need for a way to determine the role of position, and the severity of oxygen desaturations in a convenient way. This has application for the undiagnosed as well as the diagnosed, those who are adherent to PAP therapy as well as those who find PAP therapy difficult to consistently use. All would benefit from a better understanding of their OSA, the role of sleeping position, and the nightly impact of their therapy. Beyond PAP therapy, the ability to understand the efficacy of alternative treatments for OSA and the role of sleeping position therein is also of great benefit.